1. Field of the Invention
The present invention is related to an SDH device and a method of exchanging units while in service to achieve upgrading.
2. Description of the Related Art
SDH (synchronous digital hierarchy) transmission devices are arranged in STM-1 (synchronous transfer module level one) multiplexing units of 155.52 Mbps, with STM-N integral multiples of this multiplexing unit being standardized as STM-1, STM-4, and STM-16, where N is 1, 4, and 16, respectively. The STM-0, having a bit rate of 51.84 Mbps, which is 1/3 that of the STM-1 corresponds to the OC-1 used in SONET (synchronous optical network). These SDH transmission devices are implemented in units with a redundant configuration, and it is desirable to be able to perform upgrades with the units remaining in service.
However, when upgrading either the above-described SDH transmission device which had been upgraded from STM-1 to STM-4 or an SDH transmission device that was made from the start for STM-4 use (600 Mbps) to use for high-speed transmission STM-16 (2.4 Gbps), it is possible for the maintenance personnel at the NE (near-end) side and FE (far-end) side to contact each other to enable simultaneous removal of respective time-slot switching sections, thereby placing a unit switching processing section into operation. With this method, there could be a skew in the timing of the operations performed by the maintenance personnel, and it is impossible to achieve switching in the 50 ms required for in-service upgrades.